Chief executive Guy Roberts says lower international shipping costs have made imports much cheaper, driving down demand for locally produced soda ash.

"The glass industry in particular has seen reduced demand for bottles in Australia and reduced demand for glass windows in Australia," he said.

"In South Australia in particular, increasing costs and taxes have combined over the last four or five years to give the board serious cause for concern.

"We have made attempts to divest the business, and we haven't received any offers sufficiently attractive to warrant accepting them."

'Icing on the cake'

Liberal Senator Simon Birmingham has blamed the carbon tax for the company's plan to end its soda ash production.

"The real, awful truth of this is that Penrice warned this would happen. Penrice made it very clear that the carbon tax would have an adverse impact on jobs at their business. We're now seeing this come true," he said.

Senator Birmingham says it is ironic that the company will now import soda ash, boosting carbon emissions further.

"The emissions that come from importing soda ash will still be produced. They'll just be produced in a country that doesn't have a carbon tax and then that product will be transported to Australia with additional emissions in that transportation."

But Penrice says the carbon tax was not among the main reasons for its decision.

"If I describe the carbon tax as the icing on the cake, that'd be about how I feel at the moment," Mr Roberts said.

"It is not the reason that we're closing this plant, as we can all observe, there are a lot of factors at play here, but the carbon tax, on top of all these other factors, is very difficult to manage."

Environmental benefit

Mr Roberts says the the closure will have some benefits for the environment.

"Once we turn off the soda ash plant the production of calsilt will cease, as will the very expensive calsilt recovery operation," he said.

"We will also cease the requirement for ammonia and we would cease discharging ammonia into the air and into the Port River."

The company says there will be less demand for limestone from the company's mine at Angaston in the Barossa Valley but says no job losses are forecast at that site.

It says its other operations, including sodium carbonate production and a new selective salts recovery technology for the coal seam gas industry, remain profitable.

The Osborne plant will continue to produce quicklime for use in cement.

Free trade is the oldest argument in federal politics and the issue that literally defined the federation era but opposition exists to the TPP, courtesy of the Investor-State Dispute Resolutions clause.